Conventional multi-color printing presses generally have a large paper carrying drum which rotates past a plurality of printing stations. For instance, in a four color press there would be four printing stations.
One of the problems with this type of press is the difficulty in obtaining and maintaining positive gripping of the papers so as to maintain accurate registration of the four different colors.
Another problem occurs when a paper is missed or dropped and does not appear on the drum, leaving an empty space on the paper carrying drum. In that event it is necessary to retract the printing stations so that the inked rollers normally running in contact with the drum will not print on the drum. In conventional printing presses, missing papers are detected with photo-electric sensors which involve a complicated and expensive detection system.
Another problem with this type of printing press is that in conventional printing presses the printing and inking rollers at the separate printing stations are driven by the paper carrying drum. In conventional printing presses having printing fountains at the printing stations which are retracted by lifting them out of contact with the drum, the inertia of all the inking rollers limits the speed of the retracting motion. Since perfect registration of the different colors is probably the most important thing in a multi-color press it is necessary that the registration of the printing rollers be positively controlled.